1. Field of the Invention
The invention relates to a catalytic converter for the catalytic treatment of exhaust gas.
The catalytic converter is provided in particular to purify and/or to detoxify exhaust gas produced by a gasoline combustion engine of a motor vehicle, for example of an automobile, or possibly by another internal combustion engine, by means of a catalytic treatment, i.e. to free said exhaust gas from pollutants by converting them by a chemical reaction.
2. Description of the Prior Art
Known catalytic converters for the treatment of exhaust gas have a housing with an inlet and an outlet. The housing contains catalyst means having a catalyst support, frequently referred to as a substrate, with passages for the exhaust gas, or having a plurality of catalyst supports through which exhaust gas flows in succession during operation. The surfaces bounding the passages are provided with a coating which has a catalytically active material containing at least one noble metal.
Many of these known catalytic converters have the disadvantage that the exhaust gas flowing in through the inlet is distributed only over a small exhaust gas entry surface of the (first) catalyst support and/or only nonuniformly over this exhaust gas entry surface and/or that, between the inlet and the (first) catalyst support and/or in the latter, even at the beginning of operation, the exhaust gas relatively rapidly releases a large quantity of heat via the wall of the housing to the environment. If the exhaust gas is distributed only over a small exhaust gas entry surface and/or irregularly over such a surface, this increases the pressure drop or opposite pressure and furthermore causes a reduction in the efficiency and hence an increase in the required volume of the catalyst support, in the required noble metal and in the production costs. During a cold start of the internal combustion engine and catalytic converter, rapid release of a large quantity of heat by the catalytic converter means that it takes a relatively long time until the or each catalyst support reaches the temperature required for efficient exhaust gas treatment.
European Patent Disclosure 0 514 326 discloses various catalytic converters which already substantially avoid the disadvantages described above. This publication discloses, for example, catalytic converters having a housing which has a casing and catalyst means with an annular catalyst support. This consists of a packet of annular sheet metal members. The successive sheet metal members have intersecting waves. The sheet metal members are held together by retaining means. These have bolts which pass through holes in the sheet metal members and are welded at their ends to a flat plate or housing wall. An opening in the housing is connected to an inner cavity enclosed by an inner lateral surface of the catalyst support. Another opening in the housing is connected to an outer cavity present between the casing of the housing and an outer lateral surface of the catalyst support. The catalyst support has passages which run from the inner to the outer cavity and are distributed uniformly along the circumference of the outer lateral surface. The casings of the housings and the lateral surfaces of the catalyst supports of these known catalytic converters are circular or oval in cross-section and have cross-sectional dimension which are substantially larger than the axial dimensions of the casings or catalyst supports.
The annular sheet metal members of these known catalytic converters are usually produced by punching them out of quadrilateral metal sheets or out of metal strips. This gives rise to a relatively large amount of sheet metal waste, which makes the production of the catalytic converters more expensive.
There is often only a small amount of space available when catalytic converters are to be installed underneath an automobile. The space available may vary from one type of automobile to another. To permit short exhaust gas pipes, it may be desirable, for example in certain cases, for the casing to have a relatively flat cross-section and hence for a cross-sectional dimension of the casing to be relatively small compared with the amount of exhaust gas to be treated. In other cases, it might be advantageous if the casing were approximately triangular in cross-section. Furthermore, it is advantageous in certain cases if the (maximum) cross-sectional dimension of the casing is smaller than its length.
In the known catalytic converters, whose catalyst means or catalyst supports have sheet metal members and passages running from an inner to an outer cavity, the cross-sectional area of the passages increases from the inside to the outside. This may increase the amount of catalytically active noble metal required and hence the production costs.
A catalytic converter disclosed in French Patent Disclosure 2 617 903 has a housing which contains catalyst means having a packet of annular sheet metal members which have coatings containing catalytically active material. The inlet of the catalytic converter leads into a cylindrical inner cavity enclosed by the catalyst means. An outer cavity which is connected to the outlet of the catalytic converter is present between the inner surface of the housing and the catalyst means. The sheet metal members of the catalyst means are in general conical, but at least one sheet metal member of each pair of adjacent sheet metal members is provided with waves or bulges so that the sheet metal members together in pairs bound passages running from the inner cavity to the outer cavity. The sheet metal members are arranged in a cage which has an annular flange at the inlet end of the packet and, at the other end, a plate and some rods which are connected to said plate and to the annular flange and are distributed around the packet.
The length of the inner cavity of this known catalytic converter is substantially larger than its diameter. The gas flowing through the inlet into the inner cavity during operation therefore forms a jet which is deflected only on striking the plate. The flow density of the exhaust gas flowing through the passages of the catalyst means is therefore substantially lower in the vicinity of this inlet than in the vicinity of that end of the catalyst means which is opposite to the inlet. Such inhomogeneous flow through the catalyst means results in poor utilization of the catalytically active material so that the catalytic converter becomes larger and more expensive than it would have been in the case of homogeneous flow through the catalyst means. In addition, turbulence occurs in the inner cavity, increasing the pressure drop of the exhaust gas flowing through the catalytic converter. There is also a great deal of waste in the production of the conical sheet metal members from metal sheets or metal strips. Furthermore, it is expensive to form conical sheet metal members having waves or bulges. According to the application last cited, the sheet metal members can be connected to one another or to the annular flange or to the plate by spot welding. Since the sheet metal members are in contact at least partly only in the case of linear wave summits or point-like bulge peaks and furthermore have coatings, it would however be very difficult and expensive to weld all sheet metal members to one another in pairs. Moreover, in spite of such spot-type connections, the sheet metal members can still experience relatively strong deformation and move relative to one another. Since the sheet metal members are subjected to vibrations, other accelerations and temperature change during the use of the catalytic converter, there is a great danger that the sheet metal members and especially their coatings will be damaged.
A catalytic converter disclosed in French Patent Disclosure 2 075 691, which corresponds to U.S. Pat. No. 3,649,213, has a housing possessing an oval cross-section and having an inlet and an outlet. A V-shaped catalyst support arranged in the housing has two hollow limbs which are bounded on the inside by perforated inner walls and on the outside by perforated outer walls and contain a particulate material used for the catalytic treatment of the exhaust gas. The inlet leads into an inner cavity present between the perforated inner walls of the catalyst support. An outer cavity present between the perforated outer walls of the catalyst support and the inner surface sections of the housing is connected to the outlet.
The particles of the particulate material are not described in more detail in the last-cited publication but presumably consist of a ceramic core which is coated with catalytically active material. If a catalytic converter is connected to an internal combustion engine of an automobile, vibrations produced by the engine when the automobile is operated act on the catalytic converter. The particles of the particulate material are therefore agitated, and they impact against one another and against the walls of the catalyst support and also slide over one another and over the walls. The particulate material is therefore destroyed in a short time due to abrasion and fracture during the use of the catalytic converter.
The diameter of the mouth of the inlet of the catalytic converter disclosed in the last-cited publication, which mouth leads into the inner cavity, is substantially smaller than the length of the inner cavity and than the width of that section of the inner cavity which is connected to the mouth of the inlet. When the catalytic converter is used, a large part of the gas flowing at high speed from the inlet into the inner cavity therefore forms a relatively thin jet which strikes the perforated inner surface of the catalyst support only at a large axial distance away from the inlet. Considerable turbulence is produced in those regions of the inner cavity which are located to the side of the mouth of the inlet. The flow density of the exhaust gas in the catalyst support is therefore very inhomogeneous. This results in poor utilization of the catalytically active material and a large pressure drop or opposite pressure. The turbulences resulting in the inner cavity additionally increase the pressure drop or opposite pressure.